Genetic factors associated with elevated carbapenem resistance in KPC-producing Klebsiella pneumoniae. Antimicrob Agents Chemother 54:4201–4207

In the United States, the most prevalent mechanism of carbapenem resistance among Enterobacteriaceae is the production of a Klebsiella pneumoniae carbapenemase (KPC). KPC-producing isolates often exhibit a range of carbapenem MICs. To better understand the factors that contribute to overall carbapenem resistance, we analyzed 27 KPC-producing K. pneumoniae isolates with different levels of carbapenem resistance, 11 with low-level (i.e., meropenem or imipenem MIC < 4 g/ml), 2 with intermediate-level (i.e., meropenem and imipenem MIC 8 g/ml), and 14 with high-level (i.e., imipenem or meropenem MIC> 16 g/ml) carbap-enem resistance, that were received from throughout the United States. Among 14 isolates that exhibited high-level carbapenem resistance, Western blot analysis indicated that 10 produced an elevated amount of KPC. These isolates either contained an increased blaKPC gene copy number (n 3) or had deletions directly upstream of the blaKPC gene (n 7). Four additional isolates lacked elevated KPC production but had high-level carbapenem resistance. Porin sequencing analysis identified 22 isolates potentially lacking a func-tional OmpK35 and three isolates potentially lacking a functional OmpK36. The highest carbapenem MICs were found in two isolates that lacked both functioning porins and produced elevated amounts of KPC. The 11 isolates with low-level carbapenem resistance contained neither an upstream deletion nor increased blaKP